1. Field of the Invention
This invention relates to microstrip filters, and more specifically, to such filters having cascaded coupled microstrips transverse of a transmission line.
2. Description of Related Art
In most microstrip filters, the velocity inequalities between the odd and even modes are tolerated, or they can be equalized to improve harmonic responses. In these filters, the relative velocities are modified to allow the placement of stop band zeroes where they are desired.
It is known that cascaded equal length parallel microstrips shorted at one end and open at the other end, form an all-stop structure, even when they are one-quarter wavelength long. When these strips are end loaded with capacitance, as what is referred to as a comb-line filter, the pass band and stop bands separate. Typical comb-line filters use approximately one-eighth wavelength strips, and exhibit band-stop characteristics at about twice the pass band frequency.
Comb-line or similar filters are well established for use in wave guides. For instance, Hensel, in U.S. Pat. No. 3,213,382 entitled "Broadband Coupling to Comb Filter", discloses a filter formed of a plurality of conductive rods transverse to the wave guide for providing control of the upper and lower cut-off frequencies by varying the capacitances of the relevant circuits.
It is known to vary the coupling between comb elements by applying different structure to different sections of the elements. In U.S. Pat. No. 3,582,841 entitled "Ladder Line Elliptic Function Filter", Rhodes discloses changing the size and/or relative position of the parallel elements between connected sections. This approach is also developed in different ways by Makimoto et al. in U.S. Pat. No. 4,224,587 entitled "Comb-line Bandpass Filter", Rhodes in U.S. Pat. No. 3,525,954 entitled "Stepped Digital Filter", and by Levy et al. in U.S. Pat. No. 3,617,954 entitled "Semilumped Comb Line Filter".
Young et al., in U.S. Pat. No. 4,488,130 entitled "Microwave Integrated Circuit, Bandpass Filter" discloses the use of resonators extending from a wave guide, which resonators have sections with different shapes.
Other types of microstrip filters are also well known. Hattersley, in U.S. Pat. No. 2,915,716 entitled "Microstrip Filters", discloses the use of a plurality of tabs disposed along a conductor, with stubs extending from one end of the tabs connected to a ground-plane conductor. Vinding, in U.S. Pat. No. 3,566,315 entitled "Strip Line Electrical Filter Element", discloses using the capacitance of opposing fan-shaped conductive elements distributed along a conductor relative to a ground plane as a filter. Ishikawa in Japanese Pat. No. 62-163401 discloses omitting a narrow portion of the ground plane under tips of an interdigitated microstrip filter, which decreases the capacitance between alternate electrodes more than the capacitance between adjacent electrodes.
Cohen et al., in U.S. Pat. No. 4,467,296 entitled "Integrated Electronic Controlled Diode Filter Microwave Networks", disclose using bias-controlled diodes connected to resonant conductors to vary dynamically the filter characteristics. Similarly, in Soviet Union Pat. No. 1465925, a narrow section of the ground plane under the open ends of resonator conductors of a comb line filter is replaced with a conductive plate spaced by a narrow gap from the ground plane. This narrow gap is spanned by diodes that are biased to control the band-pass width of the resulting filter. This structure allows the pass band to be selected between a narrow pass-band when the diodes do not conduct to a wider pass-band when they do conduct.
The need for controlling filter characteristics, including characteristics of comb line microstrip filters, is thus well established.